1. Field of the Invention
The present invention relates to a control method for command transmission and reception in a data transmission device such as a modem, and a data transmission device capable of realizing the control method. In addition, the present invention relates to a data transmission system using the data transmission device.
2. Description of the Prior Art
In a local area network, which connects a data terminal equipment to another data terminal equipment through at least two data transmission devices such as modems, various tests for checking the operation of the network, such as loop tests, are frequently executed. To execute, for example, a loop test, a data transmission device in one side sends a control command for instructing the loop test to an opposite side data transmission device.
In a conventional data transmission device such as a modem, the following methods have been used to transmit a control command to an opposite side modem.
The first method uses a so-called secondary channel. In this method, the frequency band, through which signals are transmitted, is divided into at least two bands. One of the two bands is used to transmit data and the other is used to transmit control signals such as control commands.
The second method attaches address information to command data so as to specify an opposite side device to which the control command should be transmitted. In this case, the receiving side device extracts the address information from the received signals to determine whether the command data are sent to itself.
However, the above-mentioned methods have the following problems. In fact, the first method using the secondary channel makes the bandwidth of the data transmission channel narrower since the entire frequency band must be divided into two channels. In a conventional case where a transmission speed was relatively slow, no serious problem arose due to such a narrow bandwidth. In recent years, however, a higher data transmission speed has been required. In order to increase the data transmission speed, the entire width of a transmission band of a transmission line must be used only for data transmission. It is therefore difficult to divide the transmission band into a plurality of sub-bands.
On the other hand, a strong demand exists to reduce the manufacturing cost of the devices. As a result, a trend has arisen not to provide a function to send address information to the data transmission devices from an external equipment. If the data transmission devices do not have the function to receive address information, the second method mentioned above is difficult to realize. Moreover, the possibility of setting the wrong address remains a problem in the second method.
It may be possible to provide no address information to a command. In this case, no address should be set in the data transmission devices. However, this invites a serious problem, such as dead-lock of system, when a tandem connection is adopted between two data transmission devices such as modems.